<!DOCTYPE html>
<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
@keyframes test {
  to { --x: to; }
}
#target {
  --x: underlying;
  animation: test 1s;
}
</style>
<div id="target"></div>
<script>
CSS.registerProperty({
  name: '--x',
  syntax: '*',
  inherits: false,
});

test(() => {
  target.style.animationDelay = '-0.25s';
  assert_equals(getComputedStyle(target).getPropertyValue('--x'), 'underlying', 'at 25%');

  target.style.animationDelay = '-0.75s';
  assert_equals(getComputedStyle(target).getPropertyValue('--x'), 'to', 'at 25%');
}, 'CSS Animations neutral keyframes on registered custom properties should produce the underlying value');
</script>
